An electrostatically-driven static capacitance detection type angular velocity sensor conventionally includes a weight part (or oscillator) disposed on a semiconductor substrate. The weight part is formed on the semiconductor substrate and is able to oscillate in mutually perpendicular first and second directions. The angular velocity sensor also includes driving electrodes for receiving a driving signal to oscillate the weight part periodically in the first direction, monitor electrodes for monitoring static capacitance variations based on the oscillation of the weight part in the first direction, and detection electrodes for detecting static capacitance variations resulting from oscillation of the weight part in the second direction. The static capacitance variations are caused by a Coriolis force that arises when an angular velocity about an axis orthogonal to the first and second directions acts further to the oscillation in the first direction.
FIG. 6 shows a plan view of a prior art angular velocity sensor as disclosed in JP-A-2002-162228 (Patent Document 1) and JP-A-2002-267450 (Patent Document 2). FIG. 7 shows a sectional view of the sensor along line VII—VII in FIG. 6. This angular velocity sensor 1 is made by known semiconductor fabrication technology using a SOI (Silicon On Insulator) substrate 5 of a structure made by forming an oxide film 3 on the surface of a first silicon substrate 2 and affixing a second silicon substrate 4 to this oxide film 3. Different parts are formed by slits 9 formed in the second silicon substrate 4 constituting an upper face. A weight part 8 is disposed above an opening 10 formed by partially removing the oxide film 3 and the first silicon substrate 2 supporting the second silicon substrate 4.
The weight part 8 is supported on a base part 7 surrounding the weight part 8 by way of driving beams 14a to 14d capable of spring deformation in a first direction (hereinafter called the X direction) and detection beams 13a to 13d capable of spring deformation in a second direction (hereinafter called the Y direction). Comblike electrode parts discussed below are formed where the periphery of the weight part 8 and the base part 7 face each other.
That is, there are formed driving electrodes 15a, 15b for receiving driving signals, monitor electrodes 20a to 20d for monitoring driven oscillation of the weight part 8 in the X direction and detecting it as monitor signals, and detection electrodes 17a, 17b for detecting oscillation of the weight part 8 in the Y direction as detection signals occurring when an angular velocity ω acts about a Z axis orthogonal to the X and Y directions. Also, pads 23a, 23b, 19a, 19b and 22a to 22d for wire bonding are formed on the electrodes 15a, 15b, 17a, 17b and 20a to 20d respectively.
In this angular velocity sensor 1 shown in FIG. 6, when the driving electrodes 15a, 15b receive driving signals such as, for example sinusoidal waves, the weight part 8 responsively oscillates in the X direction on the driving beams 14a to 14d. At this time, the static capacitances between the monitor electrodes 20a to 20d and the weight part 8 vary. From this variation the amplitude and phase of the oscillation of the weight part 8 are detected. The driving signals are adjusted by a control circuit (not shown).
When, on top of the oscillation of the weight part 8 in the X direction, an angular velocity w acts about the Z axis, a Coriolis force arises in the weight part 8 in the Y direction, and the weight part 8 oscillates in the Y direction on the detection beams 13a to 13d. Variations arise in the static capacitances between the detection electrodes 17a, 17b and the weight part 8 as a result of the Y-direction oscillation. The value of the applied angular velocity ω is obtained by detecting the amounts of these variations.
Now, in an angular velocity sensor 1 of the kind shown in FIG. 6, the electrodes formed in the second silicon substrate 4 are supported on the oxide film 3 on the first silicon substrate 2 as shown in FIG. 8. FIG. 8 is a schematic sectional view illustrating how the electrodes are supported.
Consequently, as shown with a dashed line in FIG. 8, coupling arises between the driving electrodes 15a, 15b and the other electrodes because of a parasitic capacitance Cp10 formed between the driving electrodes 15a, 15b and the first silicon substrate 2 and a parasitic capacitance Cp20 formed between the monitor electrodes 20a to 20d and the first silicon substrate 2 and between the detection electrodes 17a, 17b and the first silicon substrate 2.
When this sort of coupling exists, noise signals caused by the driving signals are imposed on the monitor signals produced by the monitor electrodes 20a to 20d and the detection signals produced by the detection electrodes 17a, 17b. Because these noise signals are very large compared to the monitor signals and the detection signals, the problem arises that it is not possible to accurately detect the monitor signals and detection signals generated by the actual oscillation of the weight part 8.
As a method of reducing the influence of such noise of the driving signals, various methods have been proposed. As one of these, a method has been proposed in Patent Document 1 wherein, as shown in FIG. 6, dummy electrodes 34a–34d are formed in the second silicon substrate 4 between the driving electrodes 15a, 15b and in which are formed static capacitances the same as the parasitic capacitances formed between the driving electrodes 15a, 15b and the monitor electrodes 20a to 20d and the detection electrodes 17a, 17b. Noise signals from the driving signals entering the monitor signals and the detection signals is canceled out by signals from these dummy electrodes 34a to 34d. 
Another method has been proposed in Patent Document 2 wherein, which is also shown in FIG. 6, in which opposite phase signal electrodes 35a to 35d for receiving signals opposite in phase to the driving signals are additionally formed in the second silicon substrate 4 in the proximity of the driving electrodes 15a, 15b. Noise signals entering the monitor signals and the detection signals are canceled out by signals from these opposite phase signal electrodes 35a to 35d. 
A method has been proposed in JP-A-2002-188924 (Patent Document 3) wherein, to prevent noise caused by induction arising from interconnections close to the electrodes, electrical screening interconnections are added between the interconnections to the driving electrodes 15a, 15b and the interconnections to the monitor electrodes 20a to 20d and the detection electrodes 17a, 17b. 